Looking at samples in a scanning electron microscope often involves some type of sample preparation. The most common form of sample prep is coating the sample, usually with a heavy metal. The most common metal used for sample coating is gold. The need for coating samples is taken as a given by many people, but is it always necessary? To answer this question let us take a look at the reasons for coating a sample.

The fact that about half of our readers are not biologists does not mean that they are disinterested in the fascinating current developments in the field of biology any more than the biologist are disinterested in the equally fascinating progress of materials science or physics. Popular press and even trade journals often over simplify or at best give small pieces of information. It is nice when a refereed journal gives a state of the art review. Science Magazine recently gave such a review under the title "Development Frontiers in Biology". As is customary with Science, this issue also contains editorial summaries of many of the contributions in less technically obscure language.

Part 1 of this article series (October 1994 issue) covered the performance features of various tip shapes and explained why the <100> crystal orientation is the industry standard. Part 2 (January/February 1995 issue) covered cathode mounting design and explained how the Mini Vogel Mount cathode design provides high-stability and long-lifetime performance.

LaB6 and CeB6 cathodes are used as high-brightness, high-stability, longlifetime cathodes in a variety of electron beam applications. A high-brightness source provides small spot sizes for high resolution and improved analytical results from high beam currents.

X-ray spectrometers give the capability to determine chemical element composition in electron microscopes. The semiconductor with the most experience as an x-ray detector is silicon. Silicon is the most highly developed material on earth, and has a lot of good things going for it, but for some applications we crave something with other good properties. For example, for room temperature detectors it would be best to have a semiconductor with a wider band gap. For higher resolution it would be better to have a semiconductor with a smaller band gap. For these reasons a number of other semiconductors have been developed as x-ray detectors. In this article I will talk about narrow band gap semiconductors. Next time I will discuss large band gap semiconductors.

What happens when a cup of water and two ice cubes, contained in a bowl, are placed side by side in an 800 watt microwave oven and microwaved for two minutes at 100% power? The answer may not be obvious, even to the most ardent microwave user. The water will boil and the ice will remain essentially unmelted. This dichotomy of the results may appear puzzling to the casual observer, or may explain why your food does not defrost quite evenly. To the microwave, however, it all makes sense. Water is a dielectric and absorbs microwave energy. The net result is that it heats up. The ice under the above conditions is essentially transparent to the microwave and does not heat. It would take an ice wall greater than 30 meters thick (3.4 cm for 25° C water) to protect you from the microwave energy emitted by the standard household microwave, whereas a >170 nm thick piece of aluminum foil is all that is required to reflect that same energy.

For over 5 years, here in Pathology at Tuiane, I have used color thresholding (CT) rather than monochrome thresholding (MT) for objective quantitation of histological, immunohistological, fluorescent and thick TEM sections. Technical aspects distinguishing CT from MT were recently described in detail, and this technical note represents an abbreviation of the last paper, which I recommend for those beginning to use video imaging technology. The system I use is the V150 (r) from ONCOR. Despite the advantages of today computers, data outcome, e.g., unbiased, still depends on good experimental design, strict control and repeatability. When the stain of interest is histochemically derived, my laboratory follows standard procedures found in the Handbook of Histopathological and Histochemical Techniques. When the stain is immunohistochemically derived, my laboratory checks cross reactivity of every antibody with known positive and negative control tissues besides using irrelevant antibodies or omitting the primary antibody. In addition, preabsorbing the antibody is done whenever its substrate is available.

The Georgia Microscopical Society, a non-profit organization mee'jng at the Fernbank Science Center in Atlanta, Georgia, has sponsored a Young People's Course in Microscopy for a second year. This course, open to high school students, met for 8 sessions on Saturday mornings. The course instructors were Society members from educational, governmental and private institutions who donated their time.

This adage for my students was obviously inspired by "He who steals my purse steals trash" but the lens borrower may be in for more serious trouble than the purse snatcher. This comes about because the eyepiece fit and the objective threads are standardized to a couple of standards so that the mechanical interchange of components between different optical microscopes is easily accomplished, while the incompatibility of the optical components is not usually apparent. This incompatibility can range from easily recognized blurring of the image to only slight loss of resolution which leads to inferior work or much wasted time before the problem is recognized.

Last weekend I taught a special two-day course on the identification of explosives to a young lady from San Salvador. She is a forensic mrcroscopist newly assigned to the "Bomb Squad". Her problem was to determine what explosives were used after terrorist bombings. Fortunately, some small particles of the explosive substance usually remain after a detonating. Careful examination of a bomb crater or of bomb fragments usually uncovers these tiny residues.

My problem was to teach her the microscopical characteristics of the most likely explosives she might encounter. These include common inorganic nitrates, chlorates and percholorates and less common organic (military) explosives such as TNT, RDX, HMX, PETN and tetryl.